1. Field of the Invention
The present invention relates to an electroconductive paste containing a nickel powder which can be advantageously used to form internal electrodes of a laminated ceramic capacitor, to a laminated ceramic capacitor with internal electrodes formed by using this electroconductive paste, and to a manufacturing method therefor.
2. Description of the Related Art
FIG. 1 shows a cross-sectional view of a laminated ceramic capacitor 1 to which the present invention relates.
A laminated ceramic capacitor 1 is composed of a laminated body 3 having ceramic layers 2 comprising plural laminated dielectric ceramic bodies, and first and second external electrodes 4 and 5, located respectively at the two edges facing each other on this laminated body 3.
First internal electrodes 6 and second internal electrodes 7 are located in an alternate arrangement inside the laminated body 3. The first internal electrodes 6 are formed along plural specific interfaces between the ceramic layers 2 with respective edges exposed at one of the edges of the laminated body 3 so that they are electrically connected with the first external electrode 4. The second internal electrodes 7 are formed along plural specific interfaces between the ceramic layers 2 with respective edges exposed at the other edge of the laminated body 3 so that they are electrically connected with the second external electrode 5.
For such a laminated ceramic capacitor 1, it is becoming common to use nickel as an electroconductive material for the internal electrodes 6 and 7 in order to reduce the production cost.
The laminated ceramic capacitor 1 is, for example, manufactured in the following manner.
First, plural green ceramic sheets are prepared for forming the ceramic layers 2. Electroconductive paste films for the internal electrodes 6 or 7 are formed on specific green ceramic sheets by applying an electroconductive paste containing a nickel powder and an organic vehicle by a screen printing method or the like.
Next, the plural green ceramic sheets including those having electroconductive paste films formed accordingly are stacked, pressed, and then cut, if necessary. In this way, a raw laminated body is produced which comprises plural green laminated ceramic layers, and electroconductive paste films formed along specific interfaces between these green ceramic layers.
This green laminated body is then baked in a nonoxidizing atmosphere. Accordingly, the green ceramic layers as well as the electroconductive paste films are sintered. These electroconductive paste films thus form the internal electrodes 6 and 7.
The external electrodes 4 and 5 are then formed on the outer surfaces of the sintered laminated body so that they are electrically connected with either the internal electrodes 6 or 7.
During the baking step included in the method for manufacturing the above-described laminated ceramic capacitor 1, there are occasions in which delamination occurs between the ceramic layers 2 and the internal electrodes 6 and 7. Cracking would also occur in the laminated body 3. They are mainly caused by differences in shrinkage behavior of ceramic materials contained in the ceramic layers 2, and the shrinkage behavior of nickel, or a metal contained in the internal electrodes 6 and 7, during the baking step. More specifically, they are caused by the fact that the degree of shrinkage of nickel composing the internal electrodes 6 and 7 is larger than that of the ceramic materials composing the ceramic layers 2.
In recent years, improvements toward thinner ceramic layers 2, thinner electrodes 6 and 7, and increase in the number of layers of the ceramic layers 2 as well as the internal electrodes 6 and 7 have been in progress, along with the tendency toward smaller laminated ceramic capacitors 1 with larger capacities. To make such improvements possible, it is necessary to reduce the particle size of a nickel powder contained in an electroconductive paste for the internal electrodes 6 and 7. As a result, the proportion of the internal electrodes 6 and 7 in the overall laminated body 3, or the proportion of the nickel portion in the overall laminated body 3 becomes higher, which tends to make more prominent such problems of delamination and cracking described above.
It is effective to restrict the shrinkage of the nickel powder during the baking step to solve such problems. Conventionally, addition of a ceramic oxide or an organometallic compound to the electroconductive paste used in forming electrodes 6 and 7, as well as use of a nickel powder with a nickel crystal having a large crystal particle size, is employed to achieve this purpose.
However, if a ceramic oxide or an organometallic compound is added to the electroconductive paste, the ceramic oxide or the organometallic compound forms a solid solution in the ceramic layers 2 during the baking step, and may result in an adverse effect to the electric properties of the baked laminated ceramic capacitor 1.
On the other hand, when a nickel powder having a large crystal particle size is contained in the electroconductive paste, it tends to drastically shrink during the sintering at a high temperature described above, if the particles in the nickel powder are small. This would cause, contrary to expectation, problems of delamination and cracking to occur more often.
An object of the present invention is, therefore, to provide an electroconductive paste which can solve such problems as mentioned above.
Another object of the present invention is to provide a laminated ceramic capacitor having internal electrodes formed by using the above-mentioned electroconductive paste, and a manufacturing method therefor.
The inventors of the present invention focused on the crystal particle sizes of nickel crystals contained in nickel powder and hypothesized that choosing a specific value for the crystal particle size in relation with the average particle size of the nickel powder would act as an effective means to restrict drastic shrinkage of the nickel powder during sintering. The inventors of the present invention performed intensive research to solve the above-mentioned problems and discovered a phenomenon that as the crystal particle size of a nickel crystal becomes smaller compared with the average particle size of the nickel powder, shrinkage of the nickel powder during sintering occurs more uniformly and is thus less liable to cause drastic shrinkage during sintering as compared with a case in which a crystal having a larger crystal particle size is used. The present invention has thus been achieved.
Based on this, an electroconductive paste according to the present invention has a nickel powder dispersed in an organic vehicle, wherein the nickel powder has an average particle size of about 0.5 xcexcm or less, and the crystal particle size of nickel crystals contained in each particle of the nickel powder is less than about 20% of the average particle size.
The invention is also directed to a laminated ceramic capacitor having a laminated body comprising plural laminated ceramic layers and internal electrodes located along specific interfaces between these ceramic layers, wherein the internal electrodes are obtained by baking the above-described electroconductive paste.
The invention is also directed to a method for manufacturing a laminated ceramic capacitor. In this method for manufacturing a laminated ceramic capacitor, a raw laminated body comprising plural green laminated ceramic layers, and electroconductive paste films formed along the specific interfaces between the raw ceramic layers by using the above-described electroconductive paste is prepared, the raw ceramic layers are sintered by baking the green laminated body, the electroconductive paste films are sintered at the same time to form internal electrodes for a sintered laminated body, and external electrodes are formed on the outer surfaces of the sintered laminated body so that they are electrically connected with the internal electrodes.